I was going through some of my code and I realized I don't actually know how a CursorLoader
and LoaderManager
combination used with a CursorAdapter
connect. Heres the part that I am confused in.
agendaAdapter = new MyAgendaAdapter(this, null);
makeProviderBundle(new String[] {"_id", "event_name", "start_date", "start_time",
"end_date", "end_time", "location"}, "date(?) >= start_date and date(?) <= end_date",
new String[]{getChosenDate(), getChosenDate()}, null);
getLoaderManager().initLoader(0, myBundle, MainDisplayActivity.this);
list.setAdapter(agendaAdapter);
So how does the query()
method from my custom ContentProvider
know to send it to that specific CursorAdapter
? I just don't see the connection. I understand everything else in that but what this question is on. Oh and I should mention, the code works fine.
And now, to answer your questions...
How does the query() method from my custom
ContentProvider
...?
Well, first remember that getContentResolver().query()
doesn't invoke the content provider's query
method directly. You are invoking the content resolver's query method, which parses the Uri
, determines the provider you wish to invoke, and then calls your provider's query
method.
How does the query get sent to that specific
CursorAdapter
?
I'll walk you through the process using the API Demos as an example. Note that the API demos uses a ListFragment
instead of a ListActivity
(the difference is not important in the context of this question).
First, create (and set up) the CursorAdapter
.
mAdapter = new SimpleCursorAdapter(
getActivity(),
android.R.layout.simple_list_item_2,
null,
new String[] { Contacts.DISPLAY_NAME, Contacts.CONTACT_STATUS },
new int[] { android.R.id.text1, android.R.id.text2 },
0);
After this statement is executed, the SimpleCursorAdapter
knows how it should associate the cursor data with your views. Whatever data is in the cursor's Contacts.DISPLAY_NAME
column will be associated with the view with id android.R.id.text1
, etc.
Note that you have passed a null
cursor as the third argument to the constructor. This is very important, as we have not queried any data yet (this is the LoaderManager
and CursorLoader
's job).
Next, initialize the loader.
getLoaderManager().initLoader(0, null, this);
This tells the LoaderManager
to create and start the Loader
corresponding to id 0
.
The LoaderManager
calls onCreateLoader(int id, Bundle args)
.
onCreateloader
returns a subclass of the Loader<Cursor>
interface (i.e. a CursorLoader
, in this case). This CursorLoader
will perform the initial query and will update itself when the data changes.
If your activity/fragment has more than one loader, then you'd use switch(id)
to determine the specific loader that has been instructed to begin the loading process.
The queried cursor is passed to onLoadFinished()
.
Immediately after the CursorLoader
is instantiated and returned in step 3, the CursorLoader
performs the initial query on a separate thread and a cursor is returned. When the CursorLoader
finishes the query, it returns the newly queried cursor to the LoaderManager
, which then passes the cursor to the onLoadFinished
method. From the documentation, "the onLoadFinished
method is called when a previously created loader has finished its load."
The queried data is associated with the CursorAdapter
.
mAdapter.swapCursor(data);
Note that onLoadFinished
is also typically where you update the activity/fragment's UI with the queried data. This isn't necessary in this case, as we have previously called setListAdapter(mAdapter)
. The ListFragment
knows how to use the CursorAdapter
(see step 1)... all we need to do is pass the adapter the cursor with swapCursor
, and the ListFragment
will take care of displaying the data on the screen for us.
Let me know if you have any questions (or if there are any typos, etc.).
The cursor that contains your queried data is associated with the CursorAdapter
in onLoadFinished
. This is typically done by calling mAdapter.swapCursor(data)
.